The bus caught fire after the blast, Mastung district Assistant Commissioner Shafqat Anwar said, adding 32 people were injured and hospitalized.
The bus was traveling from Taftan, on the Pakistan-Iran border, when it was attacked, Pakistan's Dawn News reported.
Provincial Home Secretary Asadur Rehman Gilani said the attacked bus was one of two buses traveling together and accompanied by security vehicles supplied by the government.
Gilani added investigators believe about 80 kilograms (176 pounds) of explosives were used in the blast.
It was not immediately clear if the incident was a suicide attack. No group has claimed responsibility for the bombing.